1. Field of the Invention
The present invention relates generally to filter circuits and more particularly to a tuning circuit for tuning continuous-time MOSFET-C filters.
2. Background Art
Continuous-time MOSFET-C filters have recently come into use. The expression continuous-time is used to distinguish the filter from other types of filters which use data sampling techniques such as switched-capacitor filtering. Continuous-time filters possess many advantages over other types of filters such as switched-capacitor and other types of sampled-data filters. However, the frequency response of continuous-time MOSFET-C filters is sensitive to fabrication process tolerances, operating temperature variations and aging.
One solution to this problem is to employ an on-chip automatic tuning circuit which provides indirect tuning of the continuous-time filter. In addition to the main continuous-time filter, the integrated circuit includes a tuning circuit. Typically, the characteristics of the tuning circuit are comparable to that of the filter so that characteristics of the tuning circuit and filter will track one another.
An exemplary conventional tuning circuit may include an on-chip resistor in the form of an MOS transistor, with the MOS transistor having the same characteristics as the MOS transistor which make up the filter circuit. The tuning circuit includes a comparison circuit which compares the resistance of the on-chip resistor formed by the MOS transistor with an external resistor. The external resistor is a highly quality resistor selected to have a low temperature coefficient. In the event of a mismatch between the two resistances, the comparison circuit will produce a control voltage which changes the MOS transistor resistance.
The control voltage produced by the tuning circuit is used to tune the associated continuous-time filter. The MOS transistors which form the continuous-time filters are ratio matched, in terms of width W and length L, to the MOS transistor of the tuning circuit. Accordingly, the various functions which would effect the MOS transistors of the filter, including fabrication process tolerances, operating temperature variations, aging and the like will also effect the MOS transistor of the tuning circuit so that the effects tend to be cancelled.
Capacitor values used in continuous-time MOSFET-C filter cannot be accurately controlled during fabrication. Accordingly, such continuous filters which utilize external reference resistors require that the filter be initially tuned externally. In addition, this approach does not compensate for large changes in filter capacitances.
If the use of an external resistance is not acceptable, it is possible to automatically tune RC products rather than just resistance values. This is accomplished utilizing an external accurate clock signal or an accurate on-chip clock if one is available. A reference circuit is produced on the basic structures as the main filter. The reference circuit can be a duplicate of the filter or a replica of one basic cell of the filter. A phase comparator compares the phase of the reference "filter" output with that of the clock. The comparator outputs a control voltage which adjusts the reference "filter" so that the phase difference is at a predetermined value. This causes the RC products within the reference filter to attain fixed, predetermined values.
The resistors and capacitors of the filter are ratio-matched to those of the reference circuit so that the RC products within the reference circuit are also stabilized, as is the frequency response characteristics of the filter. This approach automatically corrects for variations of both resistances and capacitances and requires no initial adjustment of the frequency response.
A variation of the foregoing tuning circuit could include a voltage controlled oscillator as the reference circuit. The oscillator is constructed from the same basic structures as the filter. The phase comparator compares the output of the oscillator with the clock and produces a control voltage which cause the oscillator output to track the clock. The RC products of the oscillator and filter are stabilized by the control voltage as is the frequency response of the filter.
The foregoing techniques for indirect tuning of continuous filter all process significant shortcomings. Tuning circuits, as described, which utilize external reference resistors require initial adjustments and do not compensate adequately for changes in both R and C values. Tuning circuits, as described, with external clocks, require that the clock frequency be outside the passband of the filter. Further, if the external clock reference circuit is a phase comparator, an offset in the comparator will result in a frequency tuning error. If the external clock reference circuit is a voltage controlled oscillator, the filter will not be suitable for high frequency applications since is generally difficult to build high frequency voltage controlled oscillators that are well matched to the associated filter. Further, harmonic distortion and transconductance non-linearity may shift the frequency of the oscillator unless the amplitude of the oscillator output is carefully regulated.
A still further approach for indirect automatic tuning utilizes an integrator as a reference circuit. A reference input in the form of a sine wave is applied to the input of the integrator. The output of the integrator and the input reference are both rectified and compared using a comparison circuit. The comparator produces a control voltage which changes the gain of the integrator until the magnitude of the rectified output of the integrator matches the magnitude of the rectified reference input.
The integrator is fabricated utilizing transistors and capacitors which match the transistors and capacitors of the filter. The feedback element of the integrator is a capacitor and the input element is a resistor in the form of a MOS transistor. The control voltage is applied to the gate electrode of the transistor thereby changing the gain of the integrator.
The principal disadvantage of the gain controlled integrator tuning circuit is that a relatively large number of building blocks are required. Further, the circuitry is somewhat complicated to implement in integrated circuit form.
The present invention is an indirect tuning circuit which possess the advantages of the previously-described conventional tuning circuit which utilizes a gain controlled integrators. However, the disclosed tuning circuit can be easily implemented in integrated circuit form and requires a relatively small number of building blocks. These and other advantages of the present invention will be apparent to those skilled in the art upon reading the following Detailed Description of the Invention together with the drawings.